Re: [Xen-devel] [RFC PATCH 1/8]: PVH: Basic and preparatory changes

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Fri, 2012-08-17 at 20:24 +0100, Mukesh Rathor wrote:
> On Fri, 17 Aug 2012 09:35:54 +0100
> Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> 
> > On Thu, 2012-08-16 at 01:57 +0100, Mukesh Rathor wrote:
> > 
> > > LDT (linear address, # ents) */
> > > -    unsigned long gdt_frames[16], gdt_ents; /* GDT (machine
> > > frames, # ents) */
> > > +    unsigned long gdt_frames[16], gdt_ents; /* GDT (machine
> > > frames, # ents).*
> > > +                                            * PV in HVM: it's GDTR
> > > addr/sz */
> > 
> > I'm not sure I understand this comment. What is "GDTR addr/sz" do you
> > mean that gdtframes/gdt_ents has a different semantics here?
> > 
> > Might be worthy of a union? Or finding some other way to expand this
> > struct.
> 
> In case of PVH, the field is used to send down GDTR address and size.
> perhaps better to just leave the comment out.

I think if the semantics of this field are totally different in the two
modes then I think a union is warranted.

> > > -void __init xen_arch_setup(void)
> > > +/* Normal PV domain not running in HVM container */
> > 
> > It's a bit of a shame to overload the "HVM" term this way, to mean
> > both the traditional "providing a full PC like environment" and "PV
> > using hardware virtualisation facilities".
> > 
> > Perhaps:
> >         /* Normal PV domain without PVH extensions */
> 
> Ok, HVM==Hardware Virtual Machine seems more appropriate here,

HVM in the context of Xen means more than that though, it implies a Qemu
and emulation of a complete "PC-like" environment and all of that stuff,
which is why I think it is inappropriate/confusing to be overloading it
to mean "PV with hardware assistance" too.

Xen's use of the term HVM is a bit unhelpful, exactly because it is a
broad sounding term but with a very specific meaning, but we are kind of
stuck with it.

> but I can remove the word HVM and go with 'PVH extensions'.

Please ;-)

> 
> > >  
> > >  #ifdef CONFIG_ACPI
> > >         if (!(xen_start_info->flags & SIF_INITDOMAIN)) {
> > > diff --git a/arch/x86/xen/smp.c b/arch/x86/xen/smp.c
> > > index f58dca7..cdf269d 100644
> > > --- a/arch/x86/xen/smp.c
> > > +++ b/arch/x86/xen/smp.c
> > > @@ -300,8 +300,6 @@ cpu_initialize_context(unsigned int cpu, struct
> > > task_struct *idle) gdt = get_cpu_gdt_table(cpu);
> > > -       BUG_ON((unsigned long)gdt & ~PAGE_MASK);
> > > +               ctxt->ldt_ents = 0;
> > 
> > Something odd is going on with the indentation here (and below I've
> > just noticed). I suspect lots of the changes aren't really changing
> > anything other than whitespace?
> 
> Konrad wanted just doing the indentation without code change first
> where it made sense so that further patch makes it easy to see if
> statements added.

I disagree that this is a useful way to structure a series unless the
whitespace change is in a patch of *only* whitespace changes (and even
then this would be an uncommon way to do things IMO).

But putting the whitespace changes associated with adding an if
alongside unrelated actual semantic changes in a totally different patch
is probably the most confusing and least helpful of all the possible
options!

My personal preference would be to do the indent when adding the if and
let people who want to see the differences without the indentation
change use "diff -b".

Konrad is maintainer though, so if he likes this then fine.

Ian.


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